This invention relates to sockets used for microelectronic devices. More particularly, this invention relates to self cleaning contactors used in sockets for testing microelectronic devices with ball terminals or with flat terminals. One or more embodiments of the present invention relate to a self-cleaning electrical contactor for making electrical connections to high performance microelectronic devices, for example, and without limitation, integrated circuits (“ICs”), including microprocessors, chips for peripheral functions and RAM memories.
Sockets are used widely in electronics to burn-in and test microelectronic devices. Sockets are routinely used in systems for: (a) testing electronic device performance (an assortment of socket types have been developed to connect to a device under test (“DUT”) having a wide variety of terminals and configurations), or (b) burn-in of electronic devices at elevated temperatures. A socket for burn-in or test applications will typically have a mechanical compliance that accommodates imperfections in the DUT as well as warping and non-planarity of the printed circuit board to which the socket is attached.
Prior art sockets are differentiated typically according to the type of terminals on the DUT and according to the intended end use (i.e., application). The contactors used in sockets are typically designed to make electrical connection to terminals on microelectronic devices wherein the types of device terminals contacted by sockets include pin grid arrays (“PGAs”), J-leads, gull-wing leads, dual in-line (“DIP”) leads, ball grid arrays (“BGAs”), column grid arrays (“CGAs”), flat metal pads (“LAN” grid arrays or “LGAs”), and many others. In order to provide sockets for microelectronic devices with this variety of terminals, many contactor technologies have been developed for sockets. In addition to the foregoing, further differentiation among prior art sockets refers to low insertion force (“LIF”) sockets, zero insertion force (“ZIF”) sockets, auto-load sockets, burn-in sockets, high performance test sockets, and production sockets (i.e., sockets for use in products). In further addition to the foregoing, low cost prior art sockets for burn-in and product applications typically incorporate contactors of stamped and formed springs to contact terminals on a DUT. In still further addition to the foregoing, for high pin-count prior art sockets, a cam is often used to urge device terminals laterally against corresponding contactors to make good contact to each spring while allowing a low or zero insertion force.
For specialized applications, prior art sockets have used a wide variety of contactors, including anisotropic conductive sheets, flat springs, lithographically formed springs, fuzz buttons (available from Cinch, Inc. of Lombard, Ill.), spring wires, barrel contactors, and spring forks, among others. Prior art sockets intended for applications where many test mating cycles (also referred to as socket mount-demount cycles) are required typically use spring pin contactors of the type exemplified by Pogo® spring contacts (available from Everett Charles Technologies of Pomona, Calif.). Spring probes for applications in the electronics test industry are available in many configurations, including simple pins and coaxially grounded pins. Most prior art spring probes consist of a coil spring disposed between a first post (for contacting terminals on the DUT) and a second post (for contacting contacts on a circuit board—a device under test board or “DUT board”). Spring probes are designed to undergo 500,000 insertions before failure.
Spring probe contactors used for socketing BGA packages typically use crown tips in order to provide good contact to mating solder balls. Sharp edges of the crown tip are intended to break through any surface film on mating solder balls in order to make good conductive contact to the ball. Remnants of the surface film, including solder flux, solder oxide, organic contamination, and surface films build up contamination on the crown tip, fouling of the tip and causing unreliable contact between the contactor tip and a mating solder ball. In typical operation, spring probe contactors must be cleaned by brushing residue from the crown tips after every 10,000 to 50,000 DUT insertions in order to minimize contact failures. Fouling of crown tip contactors is responsible for economic loss due to tester down time during cleaning as well as due to false rejects resulting from contact failure. Socket cleaning and contactor failure due to contamination fouling are significant caused of tester down time in a semiconductor test facility.
Spring pin sockets typically have a plurality of spring pin contactors disposed in an array of apertures formed through a dielectric holder. By way of example, a high performance, prior art test socket may incorporate a plurality of Pogo® spring contacts, each of which is held in a pin holder with an array of holes through a thin dielectric plate. The dielectric material in a high performance, prior art test socket is typically selected from a group of dimensionally stable polymer materials including: glass reinforced Torlon 5530 available from Quadrant Engineering Plastic Products, Inc. of Reading, Pa.; Vespel; Ultem 2000 available from GE Company GE Plastics of Pittsfield, Mass.; PEEK; liquid crystal polymer; and others. The individual Pogo® spring contacts are typically selected and designed for signal conduction at an impedance level of approximately fifty (50) ohms.
The recent growth in use of BGA terminals for IC packaging has resulted in use of new and varied sockets adapted to BGA terminals for increasing terminal count and area density. BGA sockets have evolved in several directions. One type involves use of a cam driven spring wire to contact the side of each ball. Spring pins or Pogo® pins have been adapted to use in BGA sockets for certain applications in which the high cost of the socket is acceptable.
Low-cost BGA sockets for mass market applications have evolved the use of stamped and formed springs that cradle each ball of the BGA and provide some measure of mechanical compliance needed to urge a spring contactor into contact with a mating ball. Variations of stamped and formed springs are configured to use two or more formed springs to grip each ball and thereby make positive electrical contact while retaining the ball mechanically. Miniaturization and density of the mechanically stamped and formed springs are limited by present capabilities to a certain minimum size. Sockets with contactors so made are limited in density by the complexity of stamping and forming very small miniaturized springs. Further, the mechanical compliance of a stamped and formed spring is typically small in a vertical direction perpendicular to a substrate of a ball contact. Because of small compliance in a vertical direction, a miniature stamped and formed spring may be unable to accommodate motion of a contactor support relative to a ball mated to it, thereby allowing vibration, mechanical shock load and forces, flexure, and the like to cause the contactor to slide over the surface of the ball and potentially lose contact. It has also been observed that repeated microscopic motion of one contact relative to a mating contact causes fritting, that is, a build-up of small particle debris that can lead to contact failure. In addition, contamination and debris may be transferred from terminals on DUTs to mating contactors, causing a build-up of contamination on surfaces of the contactors that also lead to socket failure.
Build-up of debris on the probe from dislodged surface contamination on terminals on devices under test are a significant problem with contactors of the prior art. The contamination is typically solder flux residue, plasticizer materials, particulate contamination, organic particles, and the like. Unsuccessful attempts have been made to minimize the buildup of residue on contactor probes. One class of probes attempting to remedy the problem employs a cam action to rotate a probe plunger as the plunger is depressed. A spiral track is provided either on the plunger or on a barrel confining the plunger, wherein a cam running against the spiral track rotated the plunger as it is depressed. Rotation of the plunger caused the probe tip to wipe against a terminal biased against the probe, thereby breaking through surface contamination on the terminal. Representative patents relating to relatively large probe tips are U.S. Pat. Nos. 5,009,613; 5,032,787; and 5,633,597.
Another approach to reducing the deleterious effects of debris buildup involves the use of a hollow probe with sharp points around the circumference of the probe that cut through surface contamination on a mating terminal. The sharp points are intended to penetrate and so need not wipe against the terminal in order to cut through the surface contamination. Representative patents are U.S. Pat. Nos. 6,159,056; and 6,377,059.
Many prior art sockets are intended to provide reliable and repeatable electrical contact to electrical terminals without causing damage to either. Further, the contactors of a socket must provide a low resistance connection to mating terminals over repeated insertions of devices. In a production environment, wherein contamination and debris fouls contactors and causes test failure, periodic cleaning of the contactors of a socket is a costly and time consuming necessity. Notwithstanding the introduction of various types of contactors in test sockets, repeated socket cleaning and contact failure are continuing problems. As the number and density of terminals on microelectronic devices increases, there is an urgent and growing need for a reliable miniature contactor for test probes, i.e., one that does not fail due to contamination and that does not need repeated cleaning during production testing.